1. Field of the Invention
This invention relates to computer systems and, more particularly, to methods and apparatus for controlling the output voltage produced by charge pumps used for programming and erasing flash electrically-erasable programmable read only memory (flash EEPROM) arrays.
2. History of the Prior Art
There has been a recent trend toward lowering the power requirements of portable computers. In order to reduce power consumption, much of the integrated circuitry used in personal computers is being redesigned to run at low voltage levels. The circuitry and components used in portable computers are being designed to operate at voltage levels such as 5 volts and 3.3 volts. This helps a great deal to reduce the power needs of such computers.
However, some of the features of portable computers require higher voltages. Recently, flash electrically-erasable programmable read only memory (flash EEPROM memory) has been used to store BIOS processes. This memory may be erased and reprogrammed without removing the BIOS circuitry from the computer by running a small update program when the BIOS processes are changed. However, erasing and reprogramming flash EEPROM memory requires approximately twelve volts to accomplish effectively; and the lower voltage batteries provided in personal computers are not capable of furnishing this voltage. In other electronic arrangements, charge pump circuits have been used to provide a high voltage from a lower voltage source. However, even though charge pumps have long been available which are capable of providing the voltages necessary for programming and erasing flash EEPROM memory arrays, no arrangement had been devised until recently for utilizing these charge pumps to provide the voltages needed to accomplish erasing and programming of flash EEPROM memory arrays using those positive source erase techniques which are used when 12 volts is available from an external source.
The primary reason for the failure is the universal perception that insufficient current can be generated using charge pumps to accomplish the erase process. The positive source method of erasing flash EEPROM memories draws a very substantial amount of current. However, recently it was discovered that using specially designed charge pumps sufficient current could be generated to accomplish positive source erase of flash EEPROM memory arrays. A charge pump arrangement for accomplishing positive source erase is disclosed in detail in U.S. patent application Ser. No. 08/119,719, entitled Method And Apparatus For Programming And Erasing Flash EEPROM Memory Arrays Utilizing A Charge Pump Circuit, K. Tedrow et al, filed on even date herewith, and assigned to the assignee of the present invention.
One of the problems encountered in utilizing charge pumps is that the output of a charge pump is furnished in a series of charge pulses which are stored to provide a source voltage. The charge pump generates this form of output because the output stage functions in the manner of a diode which only transfers charge to the output in the forward biased condition. This creates a voltage ripple on the output of the pump when the current requirements are high relative to the capacitance of the load. Another drawback is that power supply noise can be created because the inductance of bond wires and lead frames produces spikes in the internal supply voltage with the pulsing output current provided by the charge pump. The variation in the voltage Vcc provided may be sufficient to cause internal switching of devices. These two different problems make it desirable to attempt to smooth the output voltage provided by the charge pumps described in the copending patent application.